1. Field of the Invention
The present invention is directed to a silicon carbide product made by coating or conversion of a carbon preform as disclosed in application Ser. No. 07/153,126, filed Feb. 8, 1988. The process and apparatus disclosed in application Ser. No. 07/153,126 filed Feb. 8, 1988 readily produces silicon carbide conversions and coating in depth with almost imperceptible change in dimensions of the product.
2. Description of the Prior Art
This invention relates to silicon carbide coated products and silicon carbide converted products that are produced with a no growth characteristic by conversion of carbon preforms using the Levin process, as disclosed in U.S. Pat. Nos. 4,871,587 patent application Ser. No. 07/153,126. "No growth" here refers to an insignificant increase in outer diameter, inner diameter, or any other dimension due to the conversion process. The term "no growth" was first applied, described in importance, and circumscribed in usage by Montgomery et al, U.S. Pat. No. 2,677,627, May 1954, in applying silicon carbide coating to graphite.
Crystalline graphite and amorphous carbon possess properties of adsorption and porosity that cause them to be highly favorable materials for conversion to SiC by the Levin process. The carbons (and SiC also) are strongly wetted by liquid silicon. Graphite and amorphous carbon are intrinsically porous and permeable to liquids and gases. The porosity can vary considerably with the method of fabrication and can exceed 30 volume percent. It is the major source of density deficit (the ideal specific gravities being 2.26 for graphite and 2.1 for amorphous carbon). In conventional machineable graphite materials, the accessible pores are in the 1 to 20 micrometer-size range.
Like amorphous carbon and graphite, silicon carbide (SiC) has a low coefficient of thermal expansion, excellent thermal stress resistance, and moderately good thermal and electrical conductivity. It melts above 4800 F.
In contrast to amorphous carbon and graphite, SiC possesses an extreme hardness (Knoop hardness about 2800, next to diamond and one form of boron nitride). Hence it is extremely abrasion-resistant, even at high temperature. Its bond strength at 2500.degree. F. is about the same as at room temperature. These properties indicate important uses for SiC, well beyond the uses of amorphous carbon and graphite, especially at high temperature.
Unfortunately, SiC materials are brittle and are extremely difficult to machine. Therefore, they are difficult and costly to shape into intricate and delicate configurations. On the other hand, in the form of graphite, carbon bodies are readily machineable into intricate objects with dimensional exactitude.
It is a matter of considerable interest, then, that carbon can be converted to SiC by the Levin process in a no growth mode. For example, in the actual converting of carbon crucibles, the aforesaid process has demonstrated that it can readily produce silicon carbide conversions in depth with almost imperceptible change in dimensions of the product.
Therefore, amorphous carbon or graphite bodies -- as preforms that have already been machined -- offer a path to provide industry with SiC products that have a high degree of intricate detailing, dimensional exactitude, dimensional stability, bond strength, and abrasion resistance, even at temperatures as high as 2500 F.
For conversion to SiC by the Levin process, the starting material can be chosen form a widely available variety of carbon preform materials, whose differences in density, porosity, and permeability carry over to the finished product.
The no growth characteristic points directly to important high-temperature flight applications. These include engine turbine blade fabrication (where conversion can be carried out to considerable depth) and engine nozzles (where only surface coating need to be made).
The SiC conversion can be applied to provide delicately shaped products such a cog wheels, electrical components, and even carbon filaments, that retain original dimensions and at the same time impart superior mechanical properties. Carbon filaments (or meshes) for composite use can be converted almost entirely to SiC rather than be layered with the deposit of pyrolytic material that results from other processes.
In powder technology, SiC powders converted from carbon particles can find important applications. Examples are inclusion into aluminum matrices for enhanced stiffness and lightness and sintering into structural devices, such as high temperature heat exchangers, ceramic seals and electrical substrates.
Products of silicon carbide are especially important to the evolving semiconductor chip and solar cell technologies. SiC substrates (in place of silicon) have been successfully exposed to working temperatures as high as 650 C. and finding important uses in high-temperature, high-power, high-frequency rectifier diodes. The capability of SiC for high-temperature resistance, heat dissipation, and rapid switching suggest uses in density- packed, high-speed computers and even in hot engines as sensors. Amorphous SiC (which can probably be made from amorphous carbon powder) is finding applications in doping and modifying energy gaps to increase the efficiency of photovoltaic devices. It appears that many of the diverse intricately patterned preforms required for semiconductor chips and solar cells can be fabricated form carbon and converted to SiC by means of the Levin process. The making of requisitely thin SiC plate is included.
Both Montgomery el al and Levin teach, by means of entirely different processes, that it is unnecessary as a first step to lay down a layer of pyrolytic carbon in order to coat an amorphous carbon or graphite object with SiC. Both teach that a carbon preform can be coated in depth with little or no change in dimension. Thus both have as a prime objective the no=growth application of SiC to carbon bodies.
Montgomery et al nozzles were found to be coated with silicon carbide inside and outside to a depth of 0.020" to 0.030". However, outward surface growth of 0.002" to 0.003" on a radius was also reported, but deemed to be easily compensated for in design.
In the Levin products, both the coating and the more extensive conversion to SiC within the preform are enabled by strong wetting and permeation of amorphous carbon and graphite, and even of the newly formed SiC, by liquid silicon in the pores of the preforms. In brief, accessibility for reaction is provided by strong adsorption in the multifarious pores intrinsic to the carbon. The reaction of carbon and liquid silicon at temperatures above 1500 C. is extremely rapid to give SiC. Nothing is lost from the original carbon structure by the replacement of a C by a larger SiC. Indeed, there results a considerable filling of voids, along with an increase in density.
On the other hand, the product of the Montgomery et al process results from reactions summarized by Montgomery et al (p. 3, lines 38-39) as: EQU SiO.sub.2 +3C=SiC+2CO.
Perforce, the reaction is disruptive of the surface, since two C atoms are converted to gaseous CO and carried away from the surface. A third C atom reacts with gaseous silicon monoxide (SiO) to give SiC, which is attached to the structure. (SiO is thermodynamically more stable than gaseous SiO.sub.2 at the 2350 C. reaction temperature.) A significant increase in void space results, along with a decrease in density. Because of the loss of two carbons per conversion, bond strength and abrasion resistance, predictably, should not be as good as the products of the Levin process.
Necessarily then, the inventive products of the two processes are appreciably different in microscopic appearance, physical properties, and capabilities.
Other great difference in process reflect very disparate costs, as well as quality, of comparable finished products. These important difference are give as follows:
Montgomery et al first charges the reactor with the reactant sand (SiO.sub.2) in a reservoir separated by a buffer from the work load (the preforms to be coated) and then operates the reactor at a temperature above 2300 C. at power levels above 200 KW for two to three hours to produce a 0.02" to 0.03" SiC coating. (It can be shown that at thermodynamic equilibrium the SiO.sub.2 becomes predominantly SiO gas, which along with released O.sub.2, reacts with the carbon.)
In contrast, the Levin reactant -- silane, a halosilane or a halosilicon -- is in the gas phase and is fed in a controlled manner into the reactor during operation at a temperature within a range of only 1500.degree. C. to 1800.degree. C. at a power level of 50 KW or below. The reactant at reactor temperature is nascent elemental silicon formed by thermal decomposition or thermochemical reaction of precursor material. (Even the most stable of the precursor materials, for example silicon tetrachloride, begins to decompose at about 800.degree. C. to yield elemental silicon.)
The Levin process applies SiC coatings in depth to carbon crucibles placed inside the reactor (and to the internal graphite reactor walls themselves) in less than 10 minutes of run time at 1625.degree. C. to 1750.degree. C., as manifested by an acquired surface abrasion resistance that is characteristic of SiC. The in-depth coating is enabled by the strong wetting of amorphous carbon and graphite and newly formed silicon carbide by liquid silicon in the pores of the preform bodies. The reaction of C and Si to give SiC is extremely rapid above 1500 C.
The short time and low temperature requirements of Levin in comparison to Montgomery et al can be an important advantage over Montgomery el at in greatly reducing surface growth, since growth is a function of the reaction temperature and of the time that the preform is at reaction temperature.